1. Technical Field of Invention
The present invention relates to denitrification promoters for typical use in wastewater treatment and methods of water treatment using such denitrification promoters.
2. Prior Art
A method extensively used in the conventional wastewater treatment to perform denitrifying reactions comprises providing tanks in which aerobic and anaerobic environments are individually formed (and which are hereunder referred to as xe2x80x9can aerobic tankxe2x80x9d and xe2x80x9can anaerobic tankxe2x80x9d, respectively) and continuously supplying the anaerobic tank with methanol or other hydrogen donors to microorganisms responsible for denitrification.
Recently proposed techniques include the use of shaped polyvinyl alcohol gel or the like to achieve both nitrification and denitrification in a single tank (see, for example, Unexamined Published Japanese Patent Application (kokai) No. 314782/1998) and supporting nitrifying or denitrifying organisms per se on organic polymer gel by inclusion or other methods of immobilization with a view to achieving higher efficiency (see, for example, Unexamined Published Japanese Patent Application (kokai) No. 224076/1996).
Prior art techniques for hydrogen donors other than methanol include the use of carboxylic acids having not more than 3 carbon atoms (see Unexamined Published Japanese Patent Application (kokai)JP-A No. 126298/1994), low-molecular weight saccharides (Unexamined Published Japanese Patent Application (kokai) JP-A No. 299986/1997) and biodegradable resins (Unexamined Published Japanese Patent Application (kokai) JP-A No. 85782/1998 and the Proceedings of the 32nd Symposium on the Study of Sewage Systems, p. 433).
However, these conventional techniques have had their own problems. The shaped polyvinyl alcohol and the like in fact are not effective hydrogen donors to denitrifying organisms and the efficiency in denitrification is very low.
The nitrifying or denitrifying organisms carried on supports by inclusion or other methods of immobilization cannot be stored indefinitely for the structural reason of immobilizing the organisms and, in addition, special production techniques have been necessary.
The carboxylic acids having not more than 3 carbon atoms and the low-molecular weight saccharides have such high water solubility that they disappear too rapidly in wastewater to claim a marked difference in effectiveness from the prior art technique of continuously adding methanol and other liquid hydrogen donors.
The use of biodegradable resins has a difficulty in that the biodegradable resins that can serve as hydrogen donors to denitrifying organisms are practically limited to hydroxybutyric acid based polymers, as well as alloy compositions of starch and vinyl chloride but commercial use of these products is difficult to realize for economical reasons.
The present invention has been accomplished under these circumstances and has as the primary object providing an inexpensive technique of prolonged effectiveness that allows nitrification and denitrification to be accomplished simultaneously in water treatment facilities using a single aerobic tank (an aeration tank only) without continuous addition of methanol or other hydrogen donors.
As a result of the intensive studies they conducted in order to solve the aforementioned problems of the prior art, the present inventors found the following: using certain classes of chemical substances having specified numbers of carbon atoms and specified chemical structures gave an appreciable denitrification promoting effect that was higher than in the prior art; an even greater denitrification promoting effect was achieved by supporting those classes of specified chemical substances on another class of specified chemical substances; in the actual wastewater treatment using such classes of chemical substances, the intended denitrification promoting effect was achieved by forming those chemical substances into specified particle sizes or shapes, or enclosing them with nonwoven fabric or nets having specified values of areal density or mesh opening, or immobilizing them on strings. The present invention has been accomplished on the basis of these findings.
The denitrification promoter according to the first aspect of the invention and the method of water treatment using said denitrification promoter according to the second aspect of the invention have the following characteristic features.
A first aspect of the invention is a denitrification promoter based on a carboxylic acid having at least 6 carbon atoms which, in particular, is a straight-chained saturated monocarboxylic acid.
A second aspect of the invention is a method of water treatment in which the denitrification promoter according to the first aspect are added into an aerobic tank, an anaerobic tank or a settling tank.